1. Field of the Invention
The present invention relates to high pressure seals, and more particularly to such a seal which is formed between two moving parts, such as in a high pressure fluid pump having a cylinder and a piston which reciprocates therein.
2. Background Art
There are various applications for high pressure pumps, one being to supply very high pressure fluid (e.g. water at a pressure as high as 10,000 psi to 100,000 psi, or possibly higher) so that this water may in turn be discharged in the form of a high velocity water jet which can be used for cutting, abrading, etc. A common configuration for such a pump is to employ a reciprocating piston which operates in a high pressure cylinder to direct the fluid, generally water, to a manifold, from which the water is guided (plumbed) to a nozzle and then discharged as a high pressure jet, possibly with an abrasive material added thereto. On the pressure stroke, the plunger or piston can generate very high pressures in the chamber, sometimes well in excess of 25,000 psi, while during the intake stroke the pressure in the cylinder chamber is substantially zero or very low.
There are a number of critical problem areas associated with the design and operation of such a high pressure fluid pump assembly. One of the problems is to provide an adequate seal between the cylinder and the piston. Even though the cylinder and piston (and also some of the other components) are made of a rigid material (e.g. metal), with the very high pressures involved, there can be a certain amount of compression, expansion or other deformation of these components. Also, the seal material itself is exposed to widely fluctuating pressures, ranging all the way from the maximum pressure in the high pressure cylinder chamber during the discharge stroke to the low pressures which occur during the intake stroke of the piston. Various seal configurations appear in the prior art. Quite commonly there is a backup ring having a cylindrical through opening through which the piston extends for movement on its reciprocating path. Then there is generally an extrusion resistant, non-abrasive seal material (generally a high molecular weight plastic material) used in combination with a resilient more yielding material (e.g. an O-ring) to activate the seal under low pressure, and also possibly with other seal rings (sometimes a metal ring) to cooperate to provide overall a seal which is able to come into proper sealing engagement over the great range of pressures encountered.
Longevity of these seals is a serious problem. Even relatively high extrusion and wear resistant seal materials are subject to extrusion under very high pressures when exposed to minute open areas. Further, where there is an interface of relatively moving metallic components with the high pressures involved, wear of these components can be a problem.
One prior art seal arrangement which has recently been used to provide a metal backup ring which has a close tolerance fit with the reciprocating piston. The circumferential, radially inward surface of this backup ring has an annular groove or recesses area, and the backup ring has, forwardly and rearwardly of this recessed area, two surface portions which are in a close tolerance fit with the piston. Forwardly of the backup ring, there is an extrusion resistant seal sleeve having an annular configuration so as to fit closely against the outer surface of the piston and the inner surface of the cylinder. The rear face of the sleeve fits against the forward surface of the backup ring, and the forward portion of this seal sleeve has a radially outward circumferential groove to receive a resilient compressible O-ring that fits against the inner face of the cylinder. At the rear radially outward edge portion of the extrusion resistant seal sleeve, the surface of the seal sleeve is tapered in a somewhat frusto-conical configuration, and there is a metal seal ring having a triangular cross sectional configuration positioned in that area. This forms a static seal for the radially inward rear surface portion of the cylinder, and also for the interface of the backup ring and the cylinder.
While the above described prior art seal assembly has functioned with reasonable effectiveness in high pressure fluid intensifiers, wear and deterioration have been significant problems, and there have been various efforts to identify the nature of these problems and also to provide appropriate solutions for the same. One of the difficulties is that since the actual functioning of the seal takes place in a enclosed very high pressure environment, it is very difficult to get direct measurements and/or observation of the phenomena which take place during actual operation of the pump. Further, some of the tolerances are very small (e.g. clearances in the order of a thousandth or a few thousandths of an inch), this also making observation and obtaining meaningful data difficult. Thus, the designer of such high pressure seals is often left to his or her own resources to theorize what operating phenomena might be related to these problems and propound solutions directed at least partly on the basis of conjecture.